A prepositional phrase = preposition + object
A preposition (see list) connects its object with another word in the sentence with qualifying information about that word.
The object of a preposition may be a noun, a pronoun, or a noun phrase.
I ate lunch with Amy. (the noun 'Amy' is the object of the preposition)
I ate lunch with her. (the pronoun 'her' is the object of the preposition)
I ate lunch with a group of friends. (noun phrase 'a group of friends is the object of the preposition)
- The object of the preposition in these sentences qualifies the verb 'ate'.
Note: The prepositional phrase in the last sentence includes the prepositional phrase 'of friends', which qualifies the noun 'group'.
Don't assume that when you see one of those words listed as a preposition that it used to form a prepositional phrase. Some of those words also function as other parts of speech.
He ran down the stairs. (the preposition 'down' connects the noun 'stairs' to the verb 'ran')
He ran up the down escalator. (the word 'down' is an adjective describing the noun 'escalator')
We waited at the station until noon. (the preposition 'until' connect the noun 'noon' to the verb 'waited')
We waited at the station until the train finally came. (the word 'until' is a conjunction connecting the parts of the compound sentence)